Study On Preparation Of Uranium Monocarbide And Uranium Tetraboride Ceramic Powders And Their Basic Properties

In order to develop a feasible closed nuclear fuel cycle,Chinese Academy of Sciences proposed the Accelerator Driven Advanced Nuclear Energy System?ADANES?innovative concept.As one of the most important parts,the Accelerator Driven Recycling of Used Nuclear Fuel?ADRUF?removed some fission products and neutron poisons from used nuclear fuel via an advanced dry head-end technology,and then refabricated the rest including U,Pu and MAs into regenerated nuclear fuel.It is well-known that UO₂ ceramic fuel is the basic nuclear fuel for pressurized water reactors.However,it has some disadvantages,such as low thermal conductivity and easily embrittled.The thermal conductivity,density and U content of the new-type UC nuclear fuel are higher than that of UO₂,and UC has an excellent mutual solubility with Pu and MAs.Therefore,UC has been considered as an alternative nuclear fuel for the4^th generation nuclear reactors,eapecially for ADS.Furthermore,an attempt to cost down the replacement of UO₂ fuel assemblies,UB₄ can be used one burnable neutron poison to produce burnable absorber nuclear fuels for increasing its loading and extending its life.UB₄ also is a candidate intermediate storage form for geological disposition owing to its stability to moisture and the nonproliferation physical characteristics.In this work,two parts of researches were carried out.One is that,due to the serious segregation of multiple components by conventional carbothermal reduction and the high-temperature volatility of Pu and Am,the Pechini-type in-situ polymerizable complex method was proposed to synthesize UC ceramic powders at low temperatures using the organic matter as carbon source.The other is that boro/carbothermal reduction method and boron carbide reduction method were chosen to prepare UB₄ ceramic powders with B₄C/C and B₄C as reductants,respectively.Firstly,UC was prepared by conventional carbothermal reduction method with UO₂and carbon black as raw materials.The effects of pressure,UO₂/C molar ratio,flowing rate of Ar and holding time were systematically investigated and the main process parameters of carbothermal reduction were obtained.Next,instead of solid carbon,organic species were used as a source of fine carbon with high surface area.UC fine powders were successfully synthesized by the Pechini-type in-situ polymerizable complex technique with citric acid as a chelating agent and mannitol as a polymerization agent.It consisted of the aqueous process and heat-treatment process.The occurrence of polyesterification between UO₂²⁺-CA chelated complexes and mannitol resulted in the immobilization of UO₂²⁺in the polymer precursor,achieving molecular-scale mixing of carbon and uranium sources.In-situ charring led to the intimate contact between UO₂ and carbon in pyrolysed products.SEM and TEM analyses exhibited that UO₂ nanoparticles were homogeneously distributed in the carbon matrix to form UO₂/C nanocomposites.Owing to a shorter diffusion distance,carbothermal reduction was performed at a lower temperature of1400°C.Then,UB₄ was successfully synthesized by boro/carbothermal reduction of UO₂ with B₄C and carbon as combined reduction agents.Excess B₄C was added to compensate the boron loss at high temperatures.Meanwhile,the researches about the oxidation resistance and corrosion resistance of UB₄ were performed.It showed better oxidation resistance than other basic uranium nuclear fuels?such as UO₂,UC,UN and U₃Si₂?,and its oxidation onset temperature was about 500°C.Its oxidation chemical process was presented as a three-step process:?1?500°C,the initial formation of U₃O₈ and B₂O₃;?2?<1000°C,the formation of UB₂O₆ by the interaction of U₃O₈ and B₂O₃;?3?>1000°C,the decomposition of UB₂O₆ to get U₃O₈.ICP-OES and SEM analyses suggested its excellent water resistance and acid-solubility.Finally,UB₄ was synthesized by boron carbide reduction of UO₂ using B₄C as a reducing agent without carbon.Exess B₄C should be added to compensate the boron loss resulting from the formation of B-rich oxides?such as B₂O₂ and B₂O?except for B₂O₃.UB₄ coarse particles?1-10?m?were observed by SEM images.The reason for coarsening may be the high-temperature induced coarsening of UO₂ particles combined with the existence of by-product B₂O₃.